Optical amplifying method

ABSTRACT

A method of amplifying a signal light includes initially multiplexing a signal light and a pumping light, initially amplifying the signal light initially demultiplexing so as to separately output the amplified signal light and the pumping light, and modifying a waveform of the amplified signal light so as to compensate for waveform distortion of the signal light along a transmission path of the signal light and providing an output of a waveform modified signal light. Thereafter, the waveform modified signal lights is secondarily multiplexed, secondarily amplified, and secondarily demultiplexed with a waveform of amplified signal light being compensated for waveform distortion of the signal light along a transmission path of the signal light an output of a waveform modified signal light. The waveform modified signal light is thirdly multiplexed and thirdly amplified so as to provide an output of amplified waveform modified signal light.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. application Ser. No. 10/270,648, filedOct. 16, 2002, now U.S. Pat. No. 6,577,439, which is a continuation ofU.S. application Ser. No. 09/585,280, Filed Jun. 2, 2000, NOW U.S. Pat.No. 6,542,290, which is a continuation of Ser. No. 09/069,772, filedApr. 30, 1998, now U.S. Pat. No. 6,091,540, which is a continuation ofU.S. application Ser. No. 08/432,074, filed May 1, 1995, now U.S. Pat.No. 5,831,754, the subject matter of which is incorporated by referenceherein.

BACKGROUND OF THE INVENTION

The present invention relates to an optical amplifier applied to anoptical transmission apparatus such as an optical transmitter/receiverapparatus or an optical repeater in an optical communication system.

In the prior art, a dispersion compensating fiber is used in an opticalreceiver or an optical transmitter in order to suppress a waveformdistortion of an optical signal due to a waveform dispersion of atransmission line fiber, but because of its large loss, it is essentialto use it with an optical amplifier for compensating the loss. Thistechnique is disclosed in OSA Optical Fiber Communication Conference,1992, pp. 367-370.

FIG. 14 shows a configuration of an optical fiber transmission systemwhich uses a prior art dispersion compensating optical transmitter 100and a prior art dispersion compensating optical receiver 200. Theoptical transmitter 100 comprises an erbium doped optical fiberamplifier 101 a, a dispersion compensating fiber 103 a and anelectro-optical converter 104. The optical receiver 200 comprises erbiumdoped optical fiber amplifiers 201 a and 201 b, optical band-passfilters 202 a and 202 b, dispersion compensating fibers 203 a and 203 band a photo-electric converter 205. Losses of the dispersioncompensating fibers used are 3.1 dB, 10.6 dB and 5.3 dB, respectivelyfor a light signal level. In order to compensate for the losses, a totalof three erbium doped optical fibers are used, which amplify the signallights by using separate pumping light sources. A characteristic of theoptical fiber amplifier when the dispersion compensating fiber is addedis that a noise figure is increased by a loss when the dispersioncompensating fiber is arranged in a preceding stage, and a light outputis decreased by the loss when the dispersion compensating fiber isarranged in a succeeding stage.

In the known dispersion compensating optical transmitter and dispersioncompensating optical receiver shown in FIG. 14, the light signal levelis lowered because of a large loss of the dispersion compensating fiber.When the optical amplifier is used to compensate for the loss, the lightoutput of the optical amplifier decreases and the problem of increase ofthe noise figure arises. In order to avoid the problem, it is necessaryto arrange a separate optical amplifier.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an optical amplifierwhich can suppress the decrease of the light output and the increase ofthe noise figure without increasing the pumping light power orincreasing the number of pumping light sources even when a lossydispersion compensation unit is used.

In order to achieve the above object, in accordance with the opticalamplifier of the present invention, an optical amplifying medium isdivided and a wavelength multiplexing/demultiplexing unit formultiplexing or demultiplexing a pumping light and a signal light isprovided in a division, and the pumping light is directly transmitted toa next stage optical amplifying medium while the signal light istransmitted to the next stage optical amplifying medium through anoptical signal characteristic compensation unit such as a dispersioncompensating fiber so that the reduction of the optical signal level dueto the loss of the optical signal characteristic compensation unit issuppressed.

In the optical amplifier of the present invention, the signal lightlevel is lowered by the passage through the optical signalcharacteristic compensation unit but it is again amplified by the nextstage optical amplifying medium by using the pumping light which is notconsumed by the preceding stage optical amplifying medium. In the nextstage optical amplifying medium, since the input light power is low, itapproaches a non-saturation state and a gain increases. As a result, itis possible to set the gain of the next stage optical amplifying mediumhigher than the loss of the optical signal characteristic compensationunit. By setting the gain of the preceding stage optical amplifyingmedium sufficiently large, the noise figure of the optical amplifier isessentially determined by the preceding stage and an effect of the lossof the inserted optical signal characteristic compensation unit to thenoise figure is suppressed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a configuration of a first embodiment of an opticalamplifier,

FIG. 2 illustrates an effect of the optical amplifier of the firstembodiment,

FIG. 3 shows a configuration of a second embodiment of the opticalamplifier,

FIG. 4 shows a configuration of a third embodiment of the opticalamplifier,

FIG. 5 shows a configuration of a fourth embodiment of the opticalamplifier,

FIG. 6 shows a configuration of a fifth embodiment of the opticalamplifier,

FIG. 7 shows a configuration of a sixth embodiment of the opticalamplifier,

FIG. 8 shows a configuration of a seventh embodiment of the opticalamplifier,

FIG. 9 shows a configuration of an eighth embodiment of the opticalamplifier,

FIG. 10 shows a configuration of a ninth embodiment of the opticalamplifier,

FIG. 11 shows a configuration of a first embodiment of an opticaltransmission system,

FIG. 12 shows a configuration of a second embodiment of an opticaltransmission system,

FIG. 13 shows a configuration of a third embodiment of an opticaltransmission system, and

FIG. 14 shows a configuration of a prior art optical transmission systemusing a dispersion compensating optical fiber in an opticaltransmitter/receiver.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the present invention is now explained in conjunctionwith the accompanying drawings. FIG. 1 shows a configuration of anoptical amplifier of a first embodiment of the present invention. Anoptical amplifier AO1 comprises rare earth doped fibers 1 a and 1 bwhich are optical amplifying media, wavelengthmultiplexing/demultiplexing unit 2 and 3, optical isolators 4 a and 4 b,a pumping light source 5 and a light signal characteristic compensationunit 6. The light signal characteristic compensation unit 6 may be ahigh dispersion fiber having a reverse dispersion property or an opticalresonator such as ethron, which compensates for the dispersion of atransmission line optical fiber, and for a wavelength multiplexingtransmission system, it may be a wavelength dependent equalizing opticalfilter having an optical amplifying medium gain. In FIG. 1, a signallight wavelength is λs shown by a solid line arrow and a pumping lightwavelength λp is shown by a broken line arrow. The signal light and thepumping light are applied to the rare earth doped optical fiber 1 athrough the optical isolator 4 a and the wavelengthmultiplexing/demultiplexing unit 2, and through the wavelengthmultiplexing/demultiplexing unit 2, respectively, so that the signallight is amplified. For a port which is not used by the wavelengthmultiplexing/demultiplexing unit 2, it is terminated by an oblique endof the optical fiber. The signal light then passes—through thewavelength multiplexing/demultiplexing unit 3, the light signalcharacteristic compensation unit 6 and the wavelengthmultiplexing/demultiplexing unit 3 in sequence, and the pumping light isapplied to a next stage rare earth doped optical fiber 1 b through onlythe wavelength multiplexing/ demultiplexing unit 3 and the signal lightis again amplified. Normally, in the rare earth doped optical fiber 1 b,a signal input is large because the signal light has already beenamplified by the rare earth doped optical fiber 1 a and it is in a gainsaturation state and the gain is low. In the configuration of thepresent embodiment, however, since only the signal light suffers fromthe loss by the light signal characteristic compensation unit 6, thesignal input is lowered and the rare earth doped optical fiber 1 bapproaches the non-saturation state and the gain rises. As a result, theloss of the light signal characteristics compensation unit 6 iscompensated by the rare earth doped optical fiber 1 b and the gain asviewed by the overall optical amplifier AO1 is not lowered by the losscompared with non-loss state of the light signal characteristiccompensation unit 6.

Referring to FIG. 2, an effect of the first embodiment is explained. Theoptical amplifier of the configuration of FIG. 1 is actually constructedand a gain and a noise figure (only a beat noise component between asignal and an amplified spontaneously emitted light is considered) areactually measured with respect to an input signal light power. A signallight wavelength is 1552 nm, and a pumping light wavelength is 980 nm.In order to examine an effect of the loss of the light signalcharacteristic compensation unit 6, optical attenuators with losses of 5dB and 10 dB are inserted. A mark □ is for the loss of 5 dB by theoptical attenuator, a mark □ is for the loss of 10 dB by the opticalattenuator and a mark □ is for non-insertion of the optical attenuator(0 dB of loss). A pumping light power applied to the rare earth dopedoptical fiber 1 a is 50 mW constant irrespective of the presence orabsence of the loss. When the input signal light power is smaller than−20 dBm, the reduction of gain for the 5 dB loss is approximately 2 dB,and the reduction of gain for the 10 dB loss is approximately 4 dB,which is less than one half of the loss. On the other hand, the noisefigure is substantially constant around 5 dB for the respective losses.The present experiment shows that, in the optical amplifier of the firstembodiment of the present invention, the loss of the light signalcharacteristic compensation unit can be compensated without increasingthe pumping light power. It further indicates that no significant changeappears in the noise figure.

FIG. 3 shows a configuration of a second embodiment of the opticalamplifier of the present invention. An optical amplifier A02 comprisesrare earth doped optical fibers 1 a and 1 b which are optical amplifyingmedia, wavelength multiplexing/demultiplexing units 2, 3 a and 3 b,optical isolators 4 a and 4 b, a pumping light source 5 and a lightsignal characteristic compensation unit 6. A signal light and a pumpinglight are applied to the rare earth doped optical fiber 1 a through theoptical isolator 4 a and the wavelength multiplexing/demultiplexing unit2 and through the wavelength multiplexing/demultiplexing unit 2,respectively, and the signal light is amplified. The signal light thenpasses through the wavelength multiplexing/demultiplexing unit 3 a, thelight signal characteristic compensation unit 6 and the wavelengthmultiplexing/demultiplexing unit 3 b in sequence, and the pumping lightis applied to the next stage rare earth doped optical fiber 1 b throughonly the wavelength multiplexing/demultiplexing units 3 a and 3 b sothat the signal light is amplified again. In the present embodiment, thesame effect as that of the first embodiment is attained.

FIG. 4 shows a configuration of a third embodiment of the opticalamplifier of the present invention. An optical amplifier A03 comprisesrare earth doped optical fibers 1 a and 1 b which are optical amplifyingmedia, wavelength multiplexing/demultiplexing units 2 and 3, opticalisolators 4 a and 4 b, a pumping light source 5 and a light signalcharacteristic compensation unit 6. A signal light is applied to therare earth doped optical fiber 1 a through the optical isolator 4 a andis amplified by the pumping light which passes through the wavelengthmultiplexing/demultiplexing unit 2, the rare earth doped optical fiber 1b and the wavelength multiplexing/demultiplexing unit 3 in sequence. Thesignal light then passes through the wavelengthmultiplexing/demultiplexing unit 3, the light signal characteristiccompensation unit 6 and the wavelength multiplexing/demultiplexing unit3 in sequence and is applied to the next stage rare earth doped opticalfiber 1 b pumped through the wavelength multiplexing/demultiplexing unit2 so that the signal light is amplified again. In the presentembodiment, the same effect as that of the first embodiment is attained.

FIG. 5 shows a configuration of a fourth embodiment of the opticalamplifier of the present invention. An optical amplifier A04 comprisesrare earth doped optical fibers 1 a and 1 b which are optical amplifyingmedia, wavelength multiplexing/demultiplexing units 2 a and 2 b, opticalisolators 4 a and 4 b, pumping light sources 5 a and 5 b and a lightsignal characteristic compensation unit 6. A signal light is applied tothe rare earth doped optical fiber 1 a through the optical isolator 4 aand the wavelength multiplexing/demultiplexing unit 2 a, and isamplified by a first pumping light (5 a) passed through the wavelengthmultiplexing/demultiplexing unit 2 a and a second pumping light (5 b)passed through the wavelength multiplexing/demultiplexing unit 2 b, therare earth doped optical fiber 1 b and the wavelengthmultiplexing/demultiplexing unit 3 in sequence. The signal light thenpasses through the wavelength multiplexing/demultiplexing unit 3, thelight signal characteristic compensation unit 6 and the wavelengthmultiplexing/demultiplexing unit 3 in sequence, and the first pumpinglight and the second pumping light are applied to the next stage rareearth doped optical fiber 1 b through the wavelengthmultiplexing/demultiplexing unit 3 and through the wavelengthmultiplexing/demultiplexing unit 2, respectively so that the signallight is amplified again. In the present embodiment, the same effect asthat of the first embodiment is attained.

FIG. 6 shows a configuration of a fifth embodiment of the opticalamplifier of the present invention. An optical amplifier A05 comprisesrare earth doped optical fibers 1 a and 1 b which are optical amplifyingmedia, wavelength multiplexing/demultiplexing units 2 a, 2 b and 3,optical isolators 4 a and 4 b, a pumping light source 5, a light signalcharacteristic compensation unit 6 and a reflection mirror 7. A signallight is applied to the rare earth doped optical fiber 1 a through theoptical isolator 4 a and the wavelength multiplexing/demultiplexing unit2 a. A pumping light is applied to the rare earth doped optical fiber 1a through the wavelength multiplexing/demultiplexing unit 2 a, and thepumping light which is not consumed in the rare earth doped opticalfiber 1 a passes through the wavelength multiplexing/demultiplexing unit3, the rare earth doped optical fiber 1 b and the wavelengthmultiplexing/demultiplexing unit 2 b and is reflected by the reflectionmirror 7, and passes through the same path and is directed to the rareearth doped optical fiber 1 a so that the signal light is amplified. Thesignal light then passes through the wavelengthmultiplexing/demultiplexing unit 3, the light signal characteristiccompensation unit 6 and the wavelength multiplexing/demultiplexing unit3 in sequence, and the signal light and the pumping light are applied tothe next stage rare earth doped optical fiber 1 b through the wavelengthmultiplexing/demultiplexing unit 3 and through the wavelengthmultiplexing/demultiplexing unit 2 b, respectively so that the signallight is amplified again. In the present embodiment, the same effect asthat of the first embodiment is attained. In the present embodiment,higher amplification effect is attained because the pumping light isreflected for utilization.

FIG. 7 shows a configuration of a sixth embodiment of the presentembodiment. An optical amplifier A06 comprises rare earth doped opticalfibers 1 a, 1 b and 1 c which are optical amplifying media, wavelengthmultiplexing/demultiplexing units 2, 3 a and 3 b, optical isolators 4 aand 4 b, a pumping light source 5 and light signal characteristiccompensation units 6 a and 6 b. A signal light is applied to the rareearth doped optical fiber 1 a through the optical isolator 4 a and thewavelength multiplexing/demultiplexing unit 2. A pumping light isapplied to the rare earth doped optical fiber 1 a through the wavelengthmultiplexing/demultiplexing unit 2 so that the signal light isamplified. The signal light then passes through the wavelengthmultiplexing/demultiplexing unit 3 a, the light signal characteristiccompensation unit 6 a and the wavelength multiplexing/demultiplexingunit in sequence, and the pumping light is applied to the rare earthdoped optical fiber 1 b through the wavelengthmultiplexing/demultiplexing unit 3 a so that the signal light isamplified again. The signal light passes through the wavelengthmultiplexing/demultiplexing unit 3 b, the light signal characteristiccompensation unit 6 b and the wavelength multiplexing/demultiplexingunit 3 b in sequence, and the pumping light is applied to the next stagerare earth doped optical fiber 1 c through the wavelengthmultiplexing/demultiplexing unit 3 b so that the signal light isamplified again. The number of optical amplifying media need not be twobut it may be three or more. In the present embodiment, the same effectas that of the first embodiment is attained. Further, in the presentembodiment, a plurality of light signal compensation units may be builtin the optical amplifier and a combined characteristic compensation maybe attained.

FIGS. 8-10 show configurations of seventh to ninth embodiments of theoptical amplifier of the present invention. The configurations aresimilar to that of the first embodiment shown in FIG. 1 except that anoptical part 4 c is arranged in an input of the light signalcharacteristic compensation unit 6 (FIG. 8), an output (FIG. 9) andinput/output (FIG. 10). The optical part 4 c may be an optical isolator,which is explained below. The light signal characteristic compensationunit 6 may be a dispersion compensating optical fiber and a reflectedlight by the Rayleigh scattering of the fiber or from an opticalconnector is returned to the optical amplifying media so that theamplification characteristic of the light signal may be deteriorated. Byinserting the optical isolator, the reflected light is suppressed. Theoptical isolator blocks the opposite direction spontaneous emissionlight traveling from the optical amplifying medium 1 b to the opticalamplifying medium 1 a. Accordingly, a higher gain and lower noiseoptical amplifier is attained.

It is now assumed that the optical part 4 c is, an optical band-passfilter. The optical band-pass filter equalizes only the light in thevicinity of the signal light and suppresses the extra spontaneousemission and amplified light outside of the signal band to enter thenext stage or preceding stage optical amplifying medium. Thus, a similarhigh gain and low noise optical amplifier is attained.

When the optical part 4 c is a complex optical part having an opticalisolator and an optical band-pass filter serially connected, the effectof the insertion of the optical isolator and the effect of the insertionof the optical band-pass filter are simultaneously attained so that ahigher gain and lower noise optical amplifier is attained.

FIG. 11 shows a configuration of a first embodiment of an opticaltransmission system using the optical amplifier of the presentinvention. It comprises an optical transmitter 100, a transmission lineoptical fiber 106, and an optical receiver 200. The optical transmitter100 comprises an electro-optical converter 104 and an optical amplifier105 having a light signal characteristic compensation unit builttherein. The optical amplifier 105 may be one of the optical amplifiersshown in the first to ninth embodiments. In accordance with the presentembodiment, the optical transmission system which can suppress thedeterioration of the gain of the optical amplifier or the noisecharacteristic due to the build-in of the light signal characteristiccompensation unit in the optical transmitter is attained.

FIG. 12 shows a configuration of a second embodiment of the opticaltransmission system using the optical amplifier of the presentinvention. It comprises an optical amplifier 100, a transmission lineoptical fiber 106 and an-optical receiver 200. The optical receiver 200comprises an optical amplifier 206 having a light signal characteristiccompensation unit built therein and a photo-electrical converter 205.The optical amplifier 206 may be one of the optical amplifiers shown inthe first to ninth embodiments. In accordance with the presentembodiment, an optical transmission system which suppresses thedeterioration of the gain of the optical amplifier or the noisecharacteristic due to the built-in of the light signal characteristiccompensation unit in the optical receiver is attained.

FIG. 13 shows a configuration of a third embodiment of the opticaltransmission system using the optical amplifier of the presentinvention. It comprises an optical transmitter 100, a transmission lineoptical fiber 106, an optical amplifying repeater 300 and an opticalreceiver 200. The optical amplifying repeater 300 may be one of theoptical amplifiers shown in the first to ninth embodiments. Inaccordance with the present embodiment, an optical transmission systemwhich suppresses the deterioration of the gain of the optical amplifieror the noise characteristic due to the built-in of the light signalcharacteristic compensation unit in the optical amplifying relay isattained.

In accordance with the present invention, the optical amplifier whichcompensates for the loss of the light signal characteristic compensationunit and suppresses the reduction of the optical output and the increaseof the noise figure without increasing the pumping light power and thenumber of pumping light sources is attained. Accordingly, the opticalamplifier of a simple and inexpensive construction having the reductionof the gain and the increase of the noise figure suppressed is attainedwhile adding a new function such as the dispersion compensation.

What is claimed is:
 1. A method of amplifying a signal light comprisingthe steps of: initially multiplexing a signal light and a pumping lightfrom a pumping light source so as to output the signal light and thepumping light; initially amplifying the signal light by utilizing thepumping light and providing an output of amplified signal light;receiving the amplified signal light and the pumping light and initiallydemultiplexing so as to separately output the amplified signal light andthe pumping light; receiving the amplified signal light and modifying awaveform of the amplified signal light so as to compensate for waveformdistortion of the signal light along a transmission path of the signallight and providing an output of a waveform modified signal light;secondarily multiplexing the waveform modified signal light which isreceived and the pumping light which is received so as to output thewaveform modified signal light and the pumping light; secondarilyamplifying the waveform modified signal light by utilizing the pumpinglight and providing an output of amplified waveform modified signallight so that a reduction of optical signal level due to loss caused bycompensation for the waveform distortion of the signal light issuppressed; receiving the amplified signal light and the pumping lightand secondarily demultiplexing so as to separately output the amplifiedsignal light and the pumping light; receiving the amplified signal lightand modifying a waveform of the amplified signal light so as tocompensate for waveform distortion of the signal light along atransmission path of the signal light and providing an output of awaveform modified signal light; thirdly multiplexing the waveformmodified signal light which is received and the pumping light which isreceived so as to output the waveform modified signal light and thepumping light; and thirdly amplifying the waveform modified signal lightby utilizing the pumping light and providing an output of amplifiedwaveform modified signal light so that a reduction of optical signallevel due to loss caused by compensation for the waveform distortion ofthe signal light is suppressed.
 2. A method of amplifying a signal lightaccording to claim 1, wherein the signal light includes light at pluralwavelength, and the step of modifying the waveform of the amplifiedsignal light includes utilizing a wavelength dependent equalizing filterand providing the output of the waveform modified signal light which iswavelength equalized.
 3. A method of amplifying a signal light accordingto claim 1, wherein the steps of initially demultiplexing andsecondarily multiplexing are performed in a single unit.
 4. A method ofamplifying a signal light according to claim 1, wherein the steps ofsecondarily demultiplexing and thirdly multiplexing are performed in asingle unit.
 5. A method of receiving a signal light comprising thesteps of: amplifying a signal light according to claim 1 includingproviding an output signal light; and receiving the output signal lightand converting the received output signal light to an electric signal.6. A method of transmission of a signal light comprising the steps of:amplifying a signal light according to claim 1 including providing anoutput signal light; and transmitting the output signal light to areceiver.
 7. A method of amplifying a signal light comprising the stepsof: initially multiplexing a signal light and a pumping light from apumping light source so as to output the signal light and the pumpinglight; initially amplifying the signal light by utilizing the pumpinglight and providing an output of amplified signal light; receiving theamplified signal light and the pumping light and initiallydemultiplexing so as to separately output the amplified signal light andthe pumping light; initially compensating for distortion of the signallight including receiving the amplified signal light and modifying awaveform of the amplified signal light so as to compensate for waveformdistortion of the signal light along a transmission path of the signallight and providing an output of a waveform modified signal light;secondarily multiplexing the waveform modified signal light which isreceived and the pumping light which is received so as to output thewaveform modified signal light and the pumping light; secondarilyamplifying the waveform modified signal light by utilizing the pumpinglight and providing an output of amplified waveform modified signallight so that a reduction of optical signal level due to loss caused bycompensation for the waveform distortion of the signal light issuppressed; receiving the amplified signal light and the pumping lightand secondarily demultiplexing so as to separately output the amplifiedsignal light and the pumping light; secondarily compensating fordistortion of the signal light including receiving the amplified signallight and modifying a waveform of the amplified signal light so as tocompensate for waveform distortion of the signal light along atransmission path of the signal light and providing an output of awaveform modified signal light; thirdly multiplexing the waveformmodified signal light which is received and the pumping light which isreceived so as to output the waveform modified signal light and thepumping light; and thirdly amplifying the waveform modified signal lightby utilizing the pumping light and providing an output of amplifiedwaveform modified signal light so that a reduction of optical signallevel due to loss caused by compensation for the waveform distortion ofthe signal light is suppressed.
 8. A method of amplifying a signal lightaccording to claim 7, wherein the signal light includes light at pluralwavelength, and the step of modifying the waveform of the amplifiedsignal light includes utilizing a wavelength dependent equalizing filterand providing the output of the waveform modified signal light which iswavelength equalized.
 9. A method of amplifying a signal light accordingto claim 7, wherein the steps of initially demultiplexing andsecondarily multiplexing are performed in a single unit.
 10. A method ofamplifying a signal light according to claim 7, wherein the steps ofsecondarily demultiplexing and thirdly multiplexing are performed in asingle unit.
 11. A method of receiving a signal light comprising thestep of: amplifying a signal light according to claim 7 includingproviding an output signal light; and receiving the output signal lightand converting the received output signal light to an electric signal.12. A method of transmission of a signal light comprising the step of:amplifying a signal light according to claim 7 including providing anoutput signal light; and transmitting the output signal light to areceiver.